Toxoplasma gondii strain T-263 is an oocyst negative genetic mutant that induces immuni in cats to wild type strains when tissue cysts or bradyzoites are administered orally. The vaccine potential of T-263 has been proven in two separate efficacy studies. The stability of the oocyst-negative phenotype of T-263 has been demonstrated. Use of this feline vaccine will reduce oocyst contamination of homes and surroundings thereby significantly reducing zoonotic toxoplasmosis in humans. Commercialization of this vaccine requires it to have a shelf life of 1-2 years. The overall objective of this proposal is to demonstrate the feasibility of stabilizing T-263 bradyzoites by lyophilization. We propose innovative approaches to the optimization of the incorporation of stabilizers and the rehydration procedure. These approaches are based on recently formulated principles of the stabilization of biological systems and utilize state- of-the-art technology. This will lead directly into the Phase II program of process scale-up and shelf life evaluation. The novel lyophilization procedure will have immediate application to the commercialization of the T-263 vaccine. The technology is likely to be appropriate for the preservation of the T. gondii ts-4 vaccine for economic animals and other organisms of economic importance.